five by eight

Hello, everyone!
In the TV series "The Walking Dead" the protagonists, some of the few survivors to a zombie plague, find a group of living, non-infected convicts locked in the prison cafeteria. They have been locked there for several months, feeding from the supplies in the neighboring pantry. The dialogue is between T-Dog, one of the rescuers, and Oscar and Axel, two of the convicts:

T-DOG: You never tried to break out of here? ​

OSCAR: Yeah, we tried to take the doors off. But if you make one peep in here, then those freaks'll be lined up outside the door growling, trying to get in. Windows got bars on there that He-Man couldn't get through. ​

AXEL: Bigger than a 5x8.

​

The "freaks" are the zombies who infested the rest of the complex. I even know who He-Man is, what I do not understand, though, is the "5x8" remark (pronounced "five by eight"). Could you, please, help me out? Thank you.

I don't know, waltern, I have never been in prison but isn't 5 feet by 8 feet too small to be a prison cell? Besides, they are in the cafeteria which is rather a large room.
As for the bars size, tomtombp, I was thinking about that, too. But what dimension might that be, 5 inches by 8 inches? Isn't it too thick for a bar?
My idea was that it could somehow refer to the He-Man: he could not get through if he were bigger than 5x8 (could it mean 5 feet 8 inches?).
But it does not make much sense, either, I am afraid.

Ouch! I never knew prison cells are that small! Since I am "spatially challenged", it is another good reason for me to obey the law
But I kind of like the idea of the index card, this is something I could work with my translation. Thank you exgerman!

... I know that the most common bars used in the US are called 2x4. ...

Click to expand...

That is not a standard U.S. size for prison bars. It's much larger than most prison bars: these are inches, not cm. This is the standard U.S. size of construction lumber: 2x4 inches (about 5x10 cm) before finishing, 1.5x3.5 inches (about 4x9 cm) as a builder would buy them.

That is correct, gramman, and I must say i was puzzled why Big Tiny (that's the con's nickname) changes the subject of their conversation. But apparently, it was Axel who gave it slightly different direction. For the rescuers "break out of here" meant "out from the prison" while for the convicts who were unaware of what was going on outside it simply meant "back to their 5 by 8 cells".
Now it makes perfect sense, thank you waltern and gramman!